In the related art, devices in which an orifice plate is provided within the pipe and the flow volume of a gas is measured by measuring a difference in pressure in front of and behind the orifice plate is known as devices that measure the flow volume of a gas flowing through the inside of a pipe. Japanese Unexamined Patent Application, First Publication No. 63-94098 (Patent Document 1) discloses a turbo compressor including a surging detecting device for measuring the flow volume of a gas and performing an operation of avoiding the occurrence of surging accompanying a reduction in flow volume.
In this surging detecting device, an orifice for flow volume measurement is attached to a suction-side pipe of the turbo compressor, a difference in pressure in front of and behind the orifice is oscillated by a pressure difference oscillator, an output signal is received by a receiver, surging is determined to have occurred, for example, if the pressure difference has a fluctuation of 10% or more as compared to a signal one second before, and a suitable surging preventing device is actuated (refer to FIG. 1 of Patent Document 1).
Japanese Unexamined Patent Application, First Publication No. 2010-236401 (Patent Document 2) discloses well-known members constituting a turbo compressor, such as an intake pipe that supplies a gas taken into a compressor impeller, and an inlet guide vane that adjusts the flow volume of the gas taken into the compressor impeller (refer to Abstract, Paragraph [0017], and FIG. 1 of Patent Document 2).
Japanese Unexamined Patent Application, First Publication No. 2002-188944 (Patent Document 3) discloses a flow measurement device in which two conduits having different internal diameters from an upstream side of the conduits to a downstream side thereof are connected to each other, an upstream pressure hole 26 and a downstream pressure hole 27 where the pressures of a gas that flows through the conduits are measured are provided on the upstream side where the internal diameter of the conduits is larger and on the downstream side where the internal diameter of the conduits are smaller, and the flow volume of the gas is measured from the difference between the pressure of the gas measured by the upstream pressure hole 26 and the pressure of the gas measured by the downstream pressure hole 27 (refer to Paragraphs [0022] and [0034] and FIG. 2 of Patent Document 3).
Japanese Unexamined Patent Application, First Publication No. 2004-197611 (Patent Document 4) discloses a well-known member constituting a turbo compressor, such as a diffuser passage 14 provided around a compressor impeller, and a scroll passage 13 provided further outside the diffuser passage 14 (refer to Paragraph [0017] and FIG. 1 of Patent Document 4).